This invention is generally directed to an improved process for preparing carrier particles, and more specifically, the present invention is directed to an improved process for oxidizing steel carrier cores, which oxidation results in carrier particles having improved triboelectric properties. In one very important aspect of the present invention, the process for producing the improved carrier particles is accomplished by effecting the oxidation reaction in the presence of water.
Numerous different methods are known for the prepartion of carrier particles useful as a component in developer compositions for electrostatographic imaging systems. Many of these processes involve treating commercially available steel particles, or beads, in an air or oxygen atmosphere at a temperature ranging from about 500 degrees Fahrenheit to about 1,000 degrees Fahrenheit. For example, these is disclosed in U.S. Pat. No. 3,923,503, the production of highly shaped, classified, low carbon steel beads, which have been heat treated to a tempered microstructure with an average hardness of 40-45 Rockwell C. In one variation of the process described in the '503 patent, carrier core materials are prepared by heat treatment in an air or oxygen atmosphere at temperatures ranging from about 500 degrees Fahrenheit to about 1000 degrees Fahrenheit, thereby providing oxidized carrier cores having a surface oxide layer comprising Fe.sub.3 O.sub.4 in a thickness of between 0.2 and 5.0 microns. According to the disclosure of this patent, the resulting oxidized low carbon steel carrier beads when subsequently coated with polymer materials, provide coated electrostatographic carrier beads having an increased coating to core adhesion, thereby resulting in longer machine life, and permitting faster machine speeds when utilizing such carrier particles.
In U.S. Pat. No. 3,767,477 there is described the preparation of oxide coated iron powders with a uniform oxide film, and having a resistance which can be selectively controlled, involving a detailed five-step process including fluidizing the iron particles in air while heating, introducing an inert gas such as nitrogen into the air, discontinuing the flow of the air but maintaining fluidization, introducing air into the inert gas and cooling the fluidized iron powder, and discontinuing the inert gas flow but maintaining fluidization in air while cooling the iron powder to a lower temperature. Carrier particles prepared in accordance with such a process apparently improve image development characteristics, and avoid fringe development.
Other processes are known for generally treating ferrous surfaces, other than iron core carrier particles. Thus, there is described in U.S. Pat. No. 2,728,696 the preparation of a strongly adherent coating consisting of hydrated ferrite oxide produced by forming on the surface involved, a film of a dilute aqueous solution of acid, or an acid reacting salt, followed by subjecting the surface to air in the presence of water vapor. Additionally, processes for treating and/or causing oxidation of various metal surfaces are described in U.S. Pat. Nos. 3,617,394, and 3,377,213.
While many methods and compositions are known for controlling the electrical properties of carrier particles, there continues to be a need for a simple and an economical process for oxidizing carrier particles wherein, for example, the electrical properties of the resulting carrier particles can be more suitably controlled over extended periods of time. Additionally, there continues to be a need for an improved process for preparing oxidized carrier particles wherein the surface texture of the resulting particles are controlled, which need is satisfied with the process of the present invention.